The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
On every body with a fluid flowing over its surface a thin friction-dominated layer, the so-called boundary-layer, within which the speed of the fluid adapts due to friction to the speed of the body, forms at the body surface. The state of the boundary-layer flow determines the drag, wherein the calm, laminar form causes a significantly smaller drag than the mixing, rough, turbulent form. The turbulent form is caused by the instability of the laminar flow form, which finally becomes turbulent in a transition process by increase of disturbances.
In order to diminish drag and, thus, to increase the efficiency of machines as well as their environmental compatibility, it is attempted to maintain the laminar form as long as possible on the surface of the body, and in the case of a wing, tail plane or rotor, maintaining the laminar form over a profile chord range as large as possible by means of transition delay. A method for achieving this object, is the suction of boundary-layer fluid at the wall surface, which stabilizes the laminar flow.
It was, however, noted that, especially in case of swept aerodynamic surfaces, the boundary-layer suction is not as efficient as desired. This is due to the fact that on such aerodynamic surfaces three-dimensional flow effects occur due to a flow component (cross flow) extending in spanwise direction. This causes an instability of (cross flow) disturbances, leading to an early transition, i.e. an early alteration of the laminar into a turbulent boundary-layer flow.
Three-dimensional laminar boundary-layer flows are highly unstable due to the existing cross flow within the boundary-layer, especially in case of pressure decrease in down-stream direction. Because this instability causes a growth of longitudinal vortex-type (primary-) disturbances and their final disintegration by unsteady secondary disturbances into the turbulent flow form, it has been attempted to weaken the cross flow by suction of boundary-layer fluid at the wall.
For this purpose the wall is provided with perforations, being mostly laser- or electron-ray-blasted micro-hole- or micro-slot arrangements. Suction chambers situated under the surface are connected with vacuum pumps, sucking the fluid from the boundary-layer. Presently, however, no aircraft with boundary-layer suction is in service, but in the past and even in the most recent past repeated flight tests were performed with suction at the vertical fin or at the wings.
For the boundary-layer suction, arrangements of micro-holes with a diameter of typically 50 μm and a chordwise/streamwise and spanwise distance between the holes of typically 500 μm-1000 μm are used, with the object of coming as close as possible to a kind of ideally porous wall. The disadvantage is, however, that each three-dimensional disturbance of the wall flow can excite exactly the aforementioned undesired vortices, even if the perforation distances are selected such that the excitation theoretically occurs in an uncritical streamwise and spanwise wave-number range. The reason is that even smallest non-uniformities in the perforation distribution may again lead to an excitation of unstable disturbances.
A possible solution is suggested in WO 03/089295 A2. Here, the perforation distribution or pattern is designed such that the excitation spectrum being obtained by means of a double-spectral-Fourier-analysis of the two-dimensional (repeating) perforation pattern, has the smallest possible amplitudes at the streamwise and spanwise wave-number values of the unstable steady vortex disturbances. Furthermore, it is proposed to design consecutive hole- or slot groups in such a way that they cancel, as far as possible, steady disturbances arriving from up-stream and being caused by the perforation, or, at least, that they dampen them in such a way that the transition is delayed in chord direction.
WO 03/089295 A2 is based on the principle of the successive local cancellation of undesired excited steady disturbances due to groups of perforations, following a linear method valid for small disturbances. This principle results typically in irregular distances of perforations in spanwise and chordwise direction within a group. In order to be able to achieve the desired minimum excitation or good cancellation, the perforations following each other in chordwise direction must be shifted in spanwise direction at least within the group relative to the streamline or the vortex axis, respectively, of the primary disturbance.
The disadvantage of this proposition is a small robustness of the method, as deviations of the ideal, calculated distribution due to manufacturing tolerances or operationally caused disturbances such as surface dirt or clogged holes lead to a strongly reduced efficiency. Furthermore, only disturbances caused by suction are minimized. There is no influence on other disturbances, e.g. due to undesired surface roughness.